1. Field of the Invention
The present invention relates to the direct gaseous reduction of metal oxides to metals at elevated temperatures below the sintering or fusion points of the metals.
2. Description of Prior Art
The direct gaseous reduction of metal oxides in iron ore to produce sponge metal is well-known in the art. Generally, known sponge metal producing processes contact a metal oxide containing ore, e.g., iron ore, with a reducing gas stream composed primarily of hydrogen and carbon monoxide at an elevated temperature below the melting point of the metal and is thus distinguishable from known blast furnace processes where the ore is melted. During such contacting, the metal oxide is reduced essentially to elemental metal with concurrent formation of porousity in metal ore particles, hence, the term "sponge metal" or "sponge iron". Although the general process has heretofore been employed for producing a wide variety of sponge metals, it has been particularly utilized on a large scale industrially for producing sponge iron as feedstock for conventional blast furnace operations.
Heretofore, known sponge metal production processes have been carried out semi-continuously in fixed bed multiple-unit reactor systems or continuously in moving bed reactor systems, usually vertical shaft moving bed reactor systems. In semi-continuous processes employing multiple reactor systems, separate bodies of the oxided iron ore material are treated in separate reactors. Conventionally, the plurality of reactors are interconnected with each other and with a source of reducing gas, comprised primarily of hydrogen and carbon monoxide. At a given period of time in operation, (1) a hot reducing gas stream is passed through at least one reactor for reducing the metal oxide (reduction stage); (2) a cooling gas stream, usually reducing gas obtained directly from a gas production source or partially depleted reducing gas recirculated from a reduction stage reactor, or mixtures thereof, is simultaneously passed through a second reactor (cooling stage); and, (3) a third reactor is being unloaded of just-produced sponge metal and loaded with fresh metal oxide-containing material for treatment (loading-unloading or production stage). During a cycle of the semi-continuous process, the gas streams are switched in a known manner to cause the cooling reactor to become the production reactor, the production reactor to become a reduction reactor and the last reduction stage reactor, to become the cooling reactor. Such switching is continued after each cycle whereby each load of metal oxide-containing material charged in a given reactor is subjected to all three stages of operation.
In conventional continuous processes employing moving bed vertical shaft reactor systems, the metal oxide-containing material is continuously passed through a vertical shaft reactor having at least a reducing zone and a cooling zone successively positioned which are respectively interconnected with sources of heated reducing gas and cooling gas streams. The preheated reducing gas is passed through the reducing zone cocurrently or countercurrently through the metal-containing material as it passes through the reducing zone for reduction of the metal oxides. Similarly, the cooling gas, usually recirculated partially spent reducing gas or cool reducing gas obtained directly from a production source, is passed through the cooling zone and reduced metal material as it passes from the reducing zone through the cooling zone.
Although the semi-continuous fixed bed multi-reactor and continuous moving bed vertical-stage reactor systems have specific operative differences, the basic processing steps are substantially the same for the production of sponge metal, i.e., (1) contacting a metal oxide-containing material with a heated reducing gas for reduction, and (2) contacting the resulting reduced sponge metal with a cooling gas for cooling. Accordingly, the problems associated with processes carried out in either or both systems are similar.
Over the years numerous processes for improving the production of sponge metal employing either and/or both multi-stage and vertical shaft reactor systems have been patented. Perhaps, the most significant and commercially successful process is that described in U.S. Pat. No. 2,900,247. The process of U.S. Pat. No. 2,900,247 is a process employing a multi-reactor system wherein the hot reducing gas used in the reduction stage is produced by combusting or burning a portion of preheated reducing gas generated by a catalytic reformer with a preheated oxygen-containing gas. As described in the patent, the combustion or burning of a portion of the reducing gas raises the temperature of the resulting gas mixture to 1800.degree. F. to 2250.degree. F. which is then passed through the metal oxide-containing ore in the reduction stage reactor to effect high conversion of the ore in a reduced time period.
As known, optimal conversion of metal oxide-containing ores to sponge metal is achieved at as high a temperature as possible below the melting point of the metal. Previously, the reducing gas had been preheated by employing indirect heat exchange superheaters and the like. However, in actual operation, such indirect heat exchange could only raise the reducing gas temperature to about 1600.degree. F. and, accordingly, left much to be desired. Hence, the process of U.S. Pat. No. 2,900,247 was a significant advancement over the state of the art at that time.
However, in actual commercial operation the basic process of U.S. Pat. No. 2,900,247 has several disadvantages, the primary disadvantage being that significant amounts of reducing components, i.e., hydrogen and carbon monoxide, are consumed during burning or combustion with the oxygen-containing gas to raise the temperature of the reducing gas. Such consumption greatly increases the amount of reducing gas required to be supplied to the system for producing a given volume of sponge metal and, accordingly, adversely affects the economics of the process.
U.S. Pat. No. 3,128,174 describes an improvement in the reducing gas heating procedure of the basic process of U.S. Pat. No. 2,900,247 wherein an oxygen-containing stream is separately preheated by combusting or burning a gaseous hydrocarbon fuel with an excess quantity of the oxygen-containing stream prior to mixing with the reducing gas stream to provide a portion of the heat required for raising the temperature of the reducing gas. The separately preheated oxygen-containing stream is then mixed with the reducing gas stream and a portion of the reducing components thereof are similarly burned or combusted. Hence, the process of U.S. Pat. No. 3,128,174 suffers from the same disadvantages of reducing gas component consumption as the process of U.S. Pat. No. 2,900,247.
U.S. Pat. No. 3,827,879 describes another supposed improvement in the basic process just described which includes establishing a closed loop reducing gas circuit and the elimination of a conventional catalytic reformer plant for supplying the reducing gas. In U.S. Pat. No. 3,827,879, the reducing gas circuit is provided by a reactor in the reduction stage of the above-described multi-reactor system, a combustion chamber connected to the reactor for preheating the reducing gas and suitable piping for recirculating partially spent reducing gas from the reactor to the combustion chamber. Methane is supplied to the reducing gas circuit and reduced sponge metal is disposed either in the reactor or the combustion chamber and is employed as a catalyst for reforming the methane to produce more reducing components. However, the reducing gas is still heated in accordance with the basic process of U.S. Pat. No. 2,900,247 by supplying oxygen to the combustion chamber for burning or combusting a portion of the reducing components of the reducing gas stream.
Another significant problem associated with the process as described in U.S. Pat. No. 2,900,247, and similar processes employing this reducing gas heating technique, is that essentially all the heat required for the reduction of the metal oxide-containing material is supplied by the heated reducing gas stream as it is passed through the ore in the reduction reactor(s) or zone(s). See also U.S. Pat. Nos. 3,136,623; 3,136,624; and 3,136,625. Significant volumes of heated reducing gas are, therefore, required to raise the ore bed temperature before efficient reduction occurs, thereby requiring increased amounts of reducing gas to be heated with the aforementioned attendant reduction component consumption problems. Moreover, the time required for high conversion of the ore in the reduction stage is quite long and, accordingly, adversely affects the overall economics for producing a given volume of sponge metal.
There are several reduction processes known which include separately preheating the metal oxide-containing ore prior to or during subjecting it to a hot reducing gas. See, for example, U.S. Pat. Nos. 3,684,486; 2,793,946; and 2,915,379, to name a few. More specifically, U.S. Pat. No. 3,684,486 describes a multi-reactor process wherein an ore bed is separately preheated prior to the conventional reduction stage with a hot non-reducing gas stream produced by combustion of a fuel, e.g., gaseous hydrocarbons, partially depleted reducing gas, etc., with at least a stoichiometric quantity of combustion air. The separate preheating of the ore bed with the hot non-reducing gas is described as improving the resulting sponge iron integrity as well as reducing overall processing time. However, this patent also describes employing the reducing gas heating procedure of the process of U.S. Pat. No. 2,900,247, for the ore reduction step and, accordingly, suffers from the same attendant disadvantages mentioned above.
U.S. Pat. No. 2,915,379 similarly discloses separately preheating the metal-containing ore by contact with combustion gas produced by combustion of a fuel with air. However, the process described in this patent also includes a reduction step wherein the reducing gas stream is heated to 1300.degree. F.-1600.degree. F. by conventional indirect heat exchange, i.e., by employment of a conventional gas heater or the like.
U.S. Pat. No. 2,793,946 discloses a preheating iron ore in a conventional vertical stage moving bed reactor system by combusting depleted reducing gas with air in the presence of the ore in the reactor. But, the preheated ore is then contacted and reduced with a reducing gas stream passed directly therethrough from a reducing gas generator without gas preheating.
Further, several patents are known which include recirculating portions of gas streams used for reduction and cooling. See, for example, U.S. Pat. Nos. 3,904,397, 3,890,142, and 3,423,201 describe such improvements in processes employing semi-continuous multi-reactor systems and U.S. Pat. Nos. 3,765,872, 3,770,421, 3,779,741, and 3,816,102 disclose such process improvements in vertical shaft moving bed reactor systems. The patents describe processes which include establishing closed loop reducing gas circuits for recirculating a portion of reducing gas effluent from the reducing reactor or zone and/or cooling gas effluent from the cooling reactor or zone for mixing with a reducing gas stream rich in reducing components, or for enrichment in a catalytic reducing gas reformer and ultimately used as the hot reducing gas stream for the reducing step. Additionally, closed loop cooling gas circuits are described wherein portions of reducing gas effluent from the reducing reactor or zone, rich reducing gas from a catalytic reformer unit and/or methane are circulated and used as the cooling gas in the respective sponge metal cooling step.
Although the processes described in the above patents do provide improved efficient use of reducing gas produced through recirculation and utilization of partially depleted reducing gas, the processes still require combusting or burning a portion of the reducing gas stream components for preheating the reducing gas passed to the reduction reactor or zone and, accordingly, suffer from the attendant disadvantages mentioned above. Moreover, in these processes, the described cooling and reducing gas circuits are tied in such that the operation of both the reduction and cooling steps are necessarily dependent upon each other. These described tie-ins significantly reduce the capability of operating any or all of the processing steps independently, if required for the production of sponge metal from varying types of metal oxide-containing materials.
An additional discussion of possible pertinent prior art is found in my co-pending application, Ser. No. 904,977 filed May 11, 1978.